1. Field of the Invention
The present invention relates to a method for polymerizing alpha-olefins to form polymers having multimodal molecular weight distribution, a catalyst used in such a method and a method for producing such a catalyst. In particular, the present invention relates to a catalyst, and a method for preparation thereof, which produces high density polyethylene (HDPE) having a multimodal molecular weight distribution. The invention is also directed to an olefin polymerization process carried out with the catalyst of the invention which produces polymers of multimodal molecular weight distribution in a single polymerization reactor under steady state polymerization conditions.
2. Description of Prior Art
Various processes have been proposed for the production of polymers having multimodal molecular weight distribution. The term "multimodal molecular weight distribution" means that two or more peaks of different molecular weights are readily discernible in a plot of molecular weight as a function of relative proportions of the portion of the polymer having the given molecular weight, such as that obtained by gel permeation chromatography (GPC) analysis of the polymer. One such process known to us utilizes tandem reactors operated in series, wherein in the first reactor the olefin is polymerized in the presence of catalyst and substantially in the absence of hydrogen as a chain transfer agent. The product is transferred to the second, downstream reactor wherein polymerization is conducted in the presence of relatively large amounts of hydrogen. The first reactor produces the high molecular weight component, and the second reactor the low molecular weight component of the final polymer product, e.g., see Morita et al, U.S. Pat. No. 4,338,424; Kuroda et al, U.S. Pat. No. 4,414,369; Raufaut, U.S. Pat. No. 4,703,094; and Kato et al, U.S. Pat. No. 4,420,592. As will be apparent to those skilled in the art, such a method of producing multimodal molecular weight distribution polymers is expensive, cumbersome, and time consuming.
It is also known that certain zirconium (Zr)-based olefin polymerization catalysts activated with an aluminum compound of the aluminoxane type produce relatively short chain length polymers even in the absence of a chain transfer agent, such as hydrogen, e.g., see Kaminsky et al, U.S. Pat. No. 4,542,199 and Sinn et al, U.S. Pat. No. 4,404,344, the entire contents of both of which are incorporated herein by reference.
Ziegler catalyst systems containing more than one transistion metals are also known, e.g., Warzelhan et al, U.S. Pat. No. 4,525,550, disclose a catalyst system containing vanadium and zirconium, and Warzelhan et al, U.S. Pat. No. 4,568,659 disclose a catalyst system containing vanadium, titanium and zirconium.
It is a primary object of the present invention to provide an olefin polymerization catalyst capable of producing polymers of multimodal molecular weight distribution in a single polymerization reactor under steady state polymerization conditions.